Composite aircraft structures offer many advantages for the commercial aircraft industry. Composite airframes can be lighter and/or stronger than airframes constructed from materials such as aluminum. Composite aircraft skin can be designed to be lightweight and flexible. As with other aircraft designs, composite aircraft structures are subjected to various forces during the operation of the aircraft. Reinforcing mechanisms are commonly used at strategic locations with respect to the composite aircraft structures to absorb and distribute these operational forces in order to maintain the structural integrity of the aircraft.
One type of reinforcing mechanism is a stiffener. A stiffener is a strip of material that transfers forces imparted on the skin to the frame in the fuselage portion of the aircraft and to the ribs, the spars in the wing portion of the aircraft, and to the beams on a bulkhead structure. Stiffeners can provide torsional rigidity, bending stiffness, and buckling resistance in composite structures for many aircraft applications. The stiffeners can allow for a reduction in the thickness of the skin, while providing a level of strength and rigidity necessary for the safe operation of the aircraft.
Conventional stiffener designs vary from manufacturer to manufacturer and from aircraft to aircraft. In a composite aircraft, stiffener designs can take the form of a hat stiffener, which may be open or closed. Other stiffener designs may include, but are not limited to, an “I” stiffener, or a “J” stiffener. In order to maintain aircraft structural integrity, the stiffener is typically designed to account for the forces described above. Conventional stiffener design and manufacturing constraints often result in sub-optimal performance for some loading conditions and potential overdesign for other conditions. For example, traditional hat stiffeners often have a constant hat web angle. Near vertical hat stiffener webs are better for interface load transfer but may need to be bolstered by stringer-end fittings that can help to mitigate torsion issues resulting from the shear lag force. A hat stiffener with a less steep hat web angle would more efficiently handle shear lag, but may need to be reinforced at pull-off interfaces with radius fillers or angle fittings.
It is with respect to these and other considerations that the disclosure made herein is presented.